Simplifies the git_ family of functions to make fish_right_prompt more
readable. git_prompt function is added, which prints the relevant git
prompt information. The git_ family of functions is moved into the
git_prompt function, and fish_right_prompt calls git_prompt
Going forward I'm trying to have modules in charge of the data for
other modules, so I moved the nnn alias call from fish to n, and added a
handy "extraFunctions" config option to fish. Maintaining the fish
module's control of the functions, but allowing others to add stuff in
